The invention relates to an optical height meter for measuring the height of a first surface of a transparent object having a reflecting second surface facing the first surface, said height meter comprising a radiation source unit for supplying a converging measuring beam whose chief ray extends at a first angle to the normal on the first surface, and a position-sensitive detection unit for detecting a radiation beam reflected by the first surface, the chief ray of said radiation beam extending at a second angle to said normal, which angle is opposite to the first angle, and a diaphragm being arranged between the first surface and the detection unit in the radiation path of the reflected radiation beam.
The invention also relates to an optical surface-inspection device and to a lithographic apparatus in which the inspection device is used for inspecting a lithographic mask whose pattern is to be projected on a substrate.
An optical height meter operating in accordance with the triangulation principle is known from, inter alia, the English-language abstract of Japanese Patent Application 5-332769 laid open to public inspection. In these types of height meters, a radiation beam is directed at an angle to the normal onto the surface to be measured, and the beam reflected by this surface is received by a position-sensitive detector or a camera, for example a CCD camera. Such a detector or camera are understood to fall within the scope of the above-mentioned term of position-sensitive detection unit. When shifting the surface in a direction perpendicular to this surface, the radiation spot formed by the reflected beam on the detector is displaced across the detector, so that the height of the surface can be determined by measuring the position of this radiation spot. In the height meter described in Japanese Patent Application 5-332769, a diaphragm is arranged in the path of the reflected beam, and a lens is placed both in front of and behind this diaphragm. The object of this arrangement is to ensure that the radiation spot formed on the surface to be measured is imaged on the detector in a 1:1 ratio under all circumstances, inter alia, at a tilt of this surface.
U.S. Pat. No. 5,359,407 describes an optical surface-inspection device which is used in the optical lithography technique for manufacturing semiconductor circuits (ICs) or liquid crystalline display panels (LCDs). A photolithographic apparatus, a wafer stepper or a wafer step-and-scanner comprising a mask table, a projection lens system and a substrate table is used with this device. The wafer stepper images a mask, which has a first pattern and is provided in the mask table, in a reduced form on a first IC area of a substrate which has a photosensitive layer and is provided in the substrate table. Subsequently, the substrate is moved with respect to the projection lens system and the mask in such a way that a second IC area comes underneath the lens system, and the mask pattern is imaged on this second area. This is continued until the mask pattern has been imaged on all IC areas of the substrate. Then, the illuminated substrate is removed from the wafer stepper so as to be developed and etched and to undergo further treatments. This substrate is then provided with a new photosensitive layer and introduced into the same or a different wafer stepper so as to be illuminated with a second mask pattern.
The details of the patterns which can be formed with a wafer stepper in a substrate have line widths of 0.5 micrometer or less. Alien particles, such as dust particles, in the radiation path may thus have a disastrous effect on the images formed. It should therefore be ensured that the apparatus itself, but also the substrates introduced into the apparatus, should be free from dust to a high degree. It is even more important that the masks are free from dust. In fact, a dust particle on a mask pattern will disturb all images formed of this pattern in the different IC areas of the substrate, so that all ICs manufactured on this substrate will exhibit the same defect. Since the details in the mask pattern are only a small factor, for example a factor of four or five, larger than those in the substrate, dust particles on the mask pattern have a great influence on the mask pattern image formed. It is therefore desirable to inspect the masks on the presence of dust particles and other contaminations before they are placed in the mask table. A very accurate and reliable mask inspection device is necessary for this purpose. As described in U.S. Pat. No. 5,359,407, a small radiation spot may be moved across the entire mask pattern for inspecting a mask surface, and the presence of a dust particle can be detected with a radiation-sensitive detector arranged in such a way that it can receive dust particle-diffused radiation of this spot. To inspect the entire upper surface of the mask pattern, a scanning radiation beam is used which scans the pattern in a first direction, in combination with a movement of the mask in a second direction perpendicular to the first direction.
The mask comprises a transparent substrate having a well-polished upper side and a mask pattern, provided on the lower side, in the form of, for example, a chromium pattern. This pattern has been pre-inspected very accurately on the presence of dust particles having small to very small dimensions of the order of 0.1 micron, and is then covered with a transparent protective layer in the form of, for example a foil. Before the mask has been introduced into the apparatus, coarser dust particles may have deposited on this foil as well as on the upper side of the mask, so that both this upper side and the foil must be inspected. To this end, two scanning inspection beams are used, one of which is directed onto the upper side of the mask and the other is directed onto the foil.
In this surface inspection, the distance, in the direction of a normal on the surface, between the detector and the surface is a critical parameter. If this distance is unequal to the desired distance, a part of the radiation specularly reflected by a dust-free area of the surface may reach the detector so that this detector supplies an erroneous signal. A reliable and sufficiently accurate height meter is therefore needed with which the height of the upper side of the mask introduced into the inspection device, and hence the distance between this surface and the detector, can be measured. This height meter supplies an electric signal with which this distance can be corrected.
When a conventional height meter is used, the problem occurs that radiation of the scanning beam reaches the lower side of the mask, where the mask pattern to be imaged is present. This mask pattern is constituted by a pattern of line-shaped apertures in an opaque layer, for example a chromium layer which reflects very well and better than the, generally glass, upper side of the mask. As a result, quite a considerable quantity of radiation coming from the lower side of the mask reaches the detector of the height meter, so that this meter supplies an erroneous signal.